A gear transmission comprising an external gear and an internal gear is disclosed in Japanese Patent Publication No. 02-107846. In the following description, Japanese Patent Publication No. 02-107846 will be called Patent Document 1. The external gear is supported on a casing. The internal gear eccentrically rotates while meshing with the external gear. An engagement hole is formed in the internal gear. The engagement hole engages with an eccentric body of a crankshaft. Torque from a motor is transmitted to the crankshaft. When the crankshaft rotates, the internal gear eccentrically rotates, and the external gear rotates relative to the casing.